The present invention relates to a semiconductor integrated circuit device in which a semiconductor power device used to achieve a high breakdown voltage and a large electric current is disposed and to equipment for a communication system which uses the semiconductor power device such as a mobile terminal or a radio base station.
A silicon carbide (SiC) having a larger band gap than silicon (Si) is a semiconductor which is high in breakdown voltage and stable even at a high temperature and is termed a power diode, a power transistor, or the like. Accordingly, an active element formed by using a SiC substrate is expected to be applied to a next-generation power device or high-temperature operating device.
In general, a power device is a generic name for a device which converts or controls high power. Exemplary applications of the power device include mobile terminal equipment functioning as a mobile station in a communication system, a car phone, or a transistor or diode disposed at a base station therein. The applications of the power device are expected to be widened in the future.
A typical modular structure used for such applications is obtained by connecting a plurality of semiconductor chips each having a power device embedded therein with wires in accordance with a use or an object and placing the connected semiconductor chips in a single package. For example, a desired circuit is constructed with semiconductor chips and wires by forming the wires on a substrate such that a circuit suitable for the use is constructed and mounting the individual semiconductor chips on the substrate. A description will be given herein below to a transmitting/receiving circuit at a radio base station which uses a Schottky diode and a MOSFET.
FIG. 20 is a block circuit diagram showing an internal structure of a conventional base station (base station in a communication system) disclosed in a document (Daisuke Ueda et al., xe2x80x9cRadio-Frequency and Optical Semiconductor Devices Exploring New Age of Data Communication,xe2x80x9d IEICE, Dec. 1, 1999, p.124). As shown in the drawing, the circuit comprises an antenna main body, a switch, a received-signal amplifier, an amplified-signal transmitter, a radio transmitter/receiver, a baseband signal processor, an interface unit, an exchange controller, a controller, and a power supply portion. The received-signal amplifier is composed of two filters and two low-noise amplifiers (LNA) disposed in series. A mixer for mixing an output from a local amplifier with an output from an RF emitter to generate an RF signal is disposed in the radio transmitter/receiver. A power dividing/synthesizing circuit having a driver amplifier, a filter, a middle amplifier, and a main amplifier disposed therein is disposed in the amplified-signal transmitter. There are further provided a baseband signal processor for processing an audio signal, an interface unit, and an exchange controller connected to a network.
At the conventional base station, the main amplifier is so configured as to perform impedance matching by disposing an input matching circuit and a field-effect transistor (MESFET) formed by using a GaAs substrate, while disposing a capacitor, an inductor, and a resistor element on each of the input side and output side.
A MOSFET formed on a silicon substrate, a diode, a capacitor, a resistor element, and the like are disposed in the controller, the baseband signal processor, the interface unit, and the exchange controller. Such parts as a capacitor and an inductor which occupy a particularly large area are formed as independent chips.
However, the foregoing conventional communication system has the following disadvantages.
At the conventional base station, the signal amplifying elements which are the most important parts of the transmitting/receiving circuit are generally formed by using the GaAs substrate. Since the heat resistance of GaAs is low, a cooling device having a high cooling ability is required to suppress a temperature rise, so that high running cost is required to maintain the base station. If the transmitting/receiving circuit is to be applied to a mobile terminal, the circuit should be scaled down. However, there are strict positional constraints such that a GaAs MESFET and the like having a low heat resistance should be positioned at a distance from a FET and an inductor of which temperatures are easily increased by an RF signal. As a result, the transmitting/receiving circuit itself is inevitably increased in size despite various considerations given to the positional relations among the individual parts.
In addition, the signal amplifying elements which are the most important parts of the transmitting/receiving circuit and the like are provided by disposing a large number of MESFETs at a portion at which a particularly large power should be amplified. As the frequency of the RF signal is higher, however, reflected waves from the MESFETs exert multiple effects so that it is difficult to achieve impedance matching. This causes the disadvantage of increased labor required by trimming for impedance adjustment.
It is therefore an object of the present invention to provide equipment for a communication system using an active element which is suitable for placement under stringent conditions including limited space.
Equipment for a communication system according to the present invention is equipment disposed in the communication system and having an active element formed by using a compound semiconductor, the active element comprising: a compound semiconductor layer provided on a substrate; and an active region provided on the compound semiconductor layer and composed of at least one first semiconductor layer functioning as a carrier flow region and at least one second semiconductor layer containing an impurity for carriers at a high concentration and smaller in film thickness than the first semiconductor layer such that the carriers are distributed therein under a quantum effect, the first and second semiconductor layers being disposed in contact with each other.
In the arrangement, the carriers in the second semiconductor layer spread out extensively to the first semiconductor layer so that the carriers are distributed in the entire active region. Since the impurity concentration is low in the first semiconductor layer during the operation of the active element, scattering by impurity ions in the first semiconductor layer is reduced. If the active element is composed of a MESFET or a Schottky diode, therefore, the carriers flow at a particularly high speed so that a large electric current is obtainable by using a low resistance. Moreover, the whole active region is depleted in the OFF state irrespective of the mean impurity concentration in the active region which is relatively high so that the carriers no more exist in the active region. Consequently, the breakdown voltage is defined by the first semiconductor layer which is low in impurity concentration so that a high breakdown voltage is obtained in the entire compound semiconductor layer.
By disposing an active element having a high breakdown voltage and a low resistance (i.e., a high current-driving power), therefore, the number of active elements in the equipment for a communication system can be reduced so that the equipment is scaled down and impedance is adjusted more easily.
The first semiconductor layer includes a plurality of first semiconductor layers and the second semiconductor layer includes a plurality of second semiconductor layers, the first semiconductor layers and the second semiconductor layers being arranged in stacked relation. The arrangement allows the foregoing effects to be exerted more positively.
The active element is the Schottky diode in a lateral configuration. The arrangement allows the Schottky diode in conjunction with the MESFET and the like to be integrated in the single substrate. In the equipment for a communication system handling an RF signal, in particular, prominent effects can be achieved such that impedance matching is performed more easily and the operating frequency is increased.
The active element is a MISFET comprising: a gate insulating film provided on the first semiconductor layer; a gate electrode provided on the gate insulating film; and source/drain regions provided on both sides of the gate electrode in the compound semiconductor layer. In the arrangement, the impurity concentration in the first semiconductor layer is low so that the number of charges of a second conductivity type trapped in the gate insulating film and in the vicinity of the interface between the gate insulating-film and the compound semiconductor layer is reduced. This lessens the interrupting effect exerted by the charges on the flow of carriers. If carriers spread under a quantum effect, charges of a first conductivity type are trapped in the impurity in the second semiconductor layer. This allows for compensation for the effect exerted on the flow of carriers by the charges of the second conductivity type trapped in the gate insulating film and in the vicinity of the interface between the gate insulating film and the compound semiconductor layer.
The equipment for a communication system further has a capacitor and an inductor provided on the compound semiconductor layer. The arrangement allows construction of an MMIC using a compound semiconductor. Since the capacitor and the inductor are integrated in the single substrate, impedance matching can be performed more easily.
The compound semiconductor layer is a SiC layer. The arrangement achieves a high breakdown voltage by using the large band gap of SiC and high degree of integration of the equipment by using a high heat resistance.
The equipment may be a base station or a mobile station in the communication system.
The communication system may be any one of a mobile phone, a PHS, a car phone, and a PDA.
The active element is disposed in a transmitter in the communication system. The arrangement allows a structure particularly suitable for a higher-power application to be used properly.
A semiconductor integrated circuit device according to the present invention is a semiconductor integrated circuit device having an active element formed by using a compound semiconductor, the active element comprising: a compound semiconductor layer provided on a substrate; and an active region provided on the compound semiconductor layer and composed of at least one first semiconductor layer functioning as a carrier flow region and at least one second semiconductor layer containing an impurity for carriers at a high concentration and smaller in film thickness than the first semiconductor layer such that the carriers are distributed therein under a quantum effect, the first and second semiconductor layers being disposed in contact with each other.
In the arrangement, the carriers in the second semiconductor layer spread out extensively to the first semiconductor layer so that the carriers are distributed in the entire active region. Since the impurity concentration is low in the first semiconductor layer during the operation of the active element, scattering by impurity ions in the first semiconductor layer is reduced. If the active element is composed of a MESFET or a Schottky diode, therefore, the carriers flow at a particularly high speed so that a large electric current is obtainable by using a low resistance. Moreover, the whole active region is depleted in the OFF state irrespective of the mean impurity concentration in the active region which is relatively high so that the carriers no more exist in the active region. Consequently, the breakdown voltage is defined by the first semiconductor layer which is low in impurity concentration so that a high breakdown voltage is obtained in the entire compound semiconductor layer.
The semiconductor integrated circuit device according to the present invention can also be implemented in preferred embodiments similarly to the foregoing equipment for a communication system.